IP Multimedia (IPMM) services provide a dynamic combination of voice, video, messaging, data, etc, within the same session. By growing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, including so-called “combinational IP Multimedia” services.
The IP Multimedia Subsystem (IMS) network (also referred to as IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Other protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP).
IMS will ease the migration from existing CS and packet-switched (PS) based access networks to all IP access networks. Based on the 3GPP standards, the IMS will serve the user as a single service engine for future PS networks. These standards also describe IMS Centralized Services (ICS) where a user's services are migrated from a CS access network to an IMS based network such as an all IP network, for example, the so-called Long Term Evolution (LTE) and LTE-Advanced systems. This means that the IMS will have to handle all originating and terminating calls.
A user equipment (UE) may comprise or represent any device used for communications. Examples of user equipment that may be used in certain embodiments of the described access networks are wireless devices such as mobile phones, terminals, smart phones, portable computing devices such as lap tops, handheld devices, tablets, net-books, computers, personal digital assistants and other wireless communication devices.
FIG. 1 illustrates schematically a communication network architecture 100 including a user equipment (UE-A) 101 in an originating network 102 and a user equipment (UE-B) 103 in a terminating network 104. When a calling party such as user A of UE-A 101 places a call to a called party such as user B of UE-B 103, the call set-up process involves an originating call associated with UE-A 101 set up in the originating network 102 and a terminating call associated with UE-B 103 set up in the terminating network 104.
The terms “originating call” and “terminating call” may comprise or represent the connection set-up signalling in relation to UE-A 101 and UE-B 103, respectively. Examples of originating or terminating calls that may be used in certain embodiments of the described network, include but are not limited to, the connection set-up signalling enabling a communication connection to be made between user A of UE-A 101 and user B of UE-B 103 in the two call halves model. The originating call is the connection set-up signalling for user A of UE-A 101 in relation to the originating network 102 in the first call half and the terminating call is the connection set-up signalling for connecting the call with user B of UE-B 103 in relation to terminating network 104 in the second call half.
The originating network 102 may include an IMS network 105, and other core and access networks such as a CS core network, PS access network, and/or a CS access network (not shown). The terminating network 104 includes an IMS core network 106, a CS core network 107, a PS access network such as LTE/LTE Advanced access network 108, a CS access network such as Wideband Code Division Multiple Access (WCDMA) access network 109, and a CS access network such as Global System for Mobile Communications (GSM) access network 110.
In this example, it is assumed that UE-A 101 is subscribed to IMS services, which include IMS voice services, messaging and video etc. When UE-A 101 places a call to UE-B 103, UE-A 101 will be the calling party and the call signalling of the first call half is the originating call in relation to UE-A 101. This will be directed to the IMS network 105 in the originating network. As UE-B 103 is located in the terminating network 104, IMS network 105 informs IMS network 106 of the terminating network 104 to proceed to set up the call signalling for the called party, which is UE-B 103, and the call signalling of the second call half, i.e. the terminating call in relation to UE-B 103. Depending on the subscription user B of UE-B 103 may have, the terminating call may be directed to/from the IMS network 106 to the CS core network 107 or to PS access networks 108 and/or 109 for connecting UE-A 101 with UE-B 103.
The IMS networks 105 and 106 may include nodes for performing a service domain selection (SDS) function, which is the selection of the service network or domain in which call services shall be executed. The choice is for call services to be executed in either the CS core network (CS domain) or the IMS. In addition, CS core network 107 also has T-SDS function for also selecting the service domain. Following SDS, if the IMS network 106 is selected, then a terminating access domain selection (T-ADS) function may be performed for selecting the access network. If the CS core network 107 is selected, then a network access selection function (e.g. a Select Access function) may be performed for selecting the access network. For example, a CS access network such as GSM access network 110 or WCDMA access network 109 and/or one or more PS access network(s) such as LTE access network 108 may be selected for delivering a terminating session or call to UE-B 103 such that a call is set up between UE-A 101 and UE-B 103.
In the case that an incoming terminating call is received through a CS core network (or domain) 107 for a user or subscriber having an IMS service, the CS core network 107 is typically required, as part of its T-SDS function, to route the incoming call to the IMS network 106. This may be implemented, for example, using Customized Applications for Mobile network Enhanced Logic (CAMEL) for call diversion to the IMS network 106, e.g. from CS network 107 to IMS network 106. As an example, upon receipt of an incoming terminating call, the gateway mobile switching center (GMSC) node (not shown) of CS network 107 may query a home subscriber server (HSS) (not shown) for routing information via a Send Routing Information (SRI) query. The user profile in the HSS is configured to return a terminating-CAMEL service indicator (T-CSI) including a Global System for Mobile communications-Service Control Function (gsmSCF) address to the GMSC node in response to the SRI query. When handling calls for a subscriber with a service provided by the IMS network, the processing at the gsmSCF (not shown) and the GMSC node results in routing of the terminating call to the IMS network using an IMS routing number (IMRN) returned from the gsmSCF.
When T-SDS is performed in the IMS network to determine the service engine for a user, the IMS network is required to receive terminating voice calls, even when a CS core network may be used as a telephony service engine for at least a sub-set of the subscribers. This may mean that a terminating call routed out from the IMS network to the CS network may be routed back to the IMS network due to the aforementioned T-SDS mechanism is used by the GMSC node. When T-CSI is retrieved by the GMSC node, then the terminating call (in which a subscriber may have CS telephony services and some IMS services) will be routed back to IMS causing a circular loop. This is because the HSS detects the subscriber is subscribed to IMS services such that the T-CSI causes the GMSC node and gsmSCF to route the terminating call to the IMS network.
As an example, in a multi-service offering where a network operator may provide both Voice over LTE (VoLTE) services to a set of users (e.g. user A of UE-A 101) and Rich Communication Suite (RCS)/RCS-email and CS telephony services to another set of users (e.g. user B of UE-B 103). For a VoLTE originating voice call from UE-A 101 to UE-B 103, the IMS network 106 of the terminating network 104 must be able to break out or route the terminating call set-up signaling to the CS core network 107. The reason for this is that when using RCS, the enriched services are handled by the IMS network 106 so that user B will have subscribed to some IMS services, but voice services of user B must be handled by the CS core network 107. Potentially, this can create a circular loop when performing T-SDS for the terminating call, as it may be routed from the IMS network 106 to the CS core network 107 and back again should the calling party be an IMS subscriber for some services, even though they may have CS telephony services.
There is a desire for a mechanism in nodes of the IMS that provide T-SDS functionality to avoid circular loops occurring between IMS and CS networks when performing a T-SDS function.